Disposable films and expanded (foamed) articles with reduced permeability to water and water vapor are useful for a variety of packaging and agricultural applications, and starch has been suggested as a component for such articles. Starch itself does not make acceptable films, although blends of starch and polyvinyl alcohol have long been known and can be formed into films with good elongation that are quickly dissolved by water. Such films have found applications, for example, as institutional laundry bags since they dissolve in the washing process. Additions to such films of a plasticizer such as glycerol are also known. See, for example, Westhoff et al., Starch-Starke, 31, pp. 163-165 (1979) and Lawton & Fanta, Carbohydrate Polymers, 23, pp. 275-280 (1994). While useful for various applications where water solubility is needed, such starch and polyvinyl alcohol films are soft, have little or no water resistance, and thus are not applicable for uses where greater structural integrity and water resistance are required.
U.S. Pat. No. 5,510,401, inventors Dehennau et al., issued Apr. 23, 1996, discusses the coating of a hydrophilic polymer, such as starch or gelatin, with a film-forming hydrophobic compound. A preferred polymer acting as a coupling agent for such films is a polyolefin modified by grafting maleic anhydride, and of ethylene copolymers and terpolymers containing units derived from maleic anhydride.
U.S. Pat. No. 3,949,145, inventors Otey et al., issued Apr. 6, 1976, describes biodegradable starch-based agricultural mulch films that are completely covered with a water resistant resin coating such as PVC. However, the water-resistant resin coating must be bonded to the starch film to prevent delamination. Accordingly, a bonding agent formed from a polyol and toluene diisocyanate is used to prevent delamination. Thus, both the preferred water resistant coating and the necessary bonding agent are not readily biodegradable so that when they deteriorate in the field there can be, over time, a build-up of synthetic polymers in the soil.
U.S. Pat. No. 4,863,655, inventors Lacourse et al., issued Sep. 5, 1989, describes the disposal problems associated with most presently used packaging materials formed from synthetic polymers. For example, although expanded polystyrene is a resilient, compressible and low density (about 0.25 lb/ft.sup.3) protective packaging filler material and performs its protective function well (e.g. as the ubiquitous "peanuts"), it is not biodegradable.
U.S. Pat. No. 5,412,005, issued May 2, 1995, inventors Bastioli et al. describes biodegradable polymeric compositions based on starch and thermoplastic polymers. However, the preferred polymers are water soluble, such as polyvinyl alcohol. Although these films are biodegradable and absorbent, they are not suitable (at least unless laminated to water insoluble films) for water resistant applications.
U.S. Pat. No. 5,095,054, issued Mar. 10, 1992, inventors Lay et al. describes shaped articles from conventional thermoplastic water-insoluble polymers and melted starch. While these blends may be usefully formed into articles for various applications, they are said to retain a surprisingly high degree of disintegration in contact with liquid water, and thus have limited usefulness with applications where moisture resistance is desired.
As a consequence, attempts continue to be made to find starch-based or starch including, self-supporting substrates that can be formed into articles, such as, for example, disposable films or foamed articles, and that have sufficient water-resistance for the intended applications yet which are biodegradable. Such articles further need to be competitive in price with commodity plastics such as polyethylene or polystyrene.